1. Field of the Invention
The invention relates to a measuring device for interferometric measurement of an optical imaging system and to a microlithographic projection exposure installation in which such a measuring device is integrated.
2. Discussion of the Related Art
Higher and higher requirements are being placed on the imaging quality of optical imaging systems. One example of this is projection objectives for microlithographic production of semiconductor components and other finely structured components which are intended to produce structures in the sub-micrometer range largely free of imaging errors. Since, on account of the complicated optical structure, it is as a rule impossible to derive the optical properties of the objectives from theoretical calculations, the optical properties have to be measured reliably, at least during the assembly and the adjustment of the optical imaging systems. It is also desirable to be able to measure the optical properties of the projection objectives during operation at the location of use, in order to be able to counteract any worsening of the imaging quality in good time.
Interferometric measuring methods are frequently used. A device for registering wavefronts which operates in the manner of a shearing interferometer and which permits fast, highly accurate measurement of extremely high resolution photolithographic projection objectives is described in the German patent application DE 101 09 929. This comprises a wavefront source for generating at least one wavefront traversing the imaging system, a diffraction grating, arranged downstream of the imaging system, for interacting with the wavefront reshaped by the imaging system, and a spatially resolving detector, arranged downstream of the diffraction grating, for acquiring interferometric information. The spatial structure of the wavefront source is used in this case to shape the spatial coherence of the wavefront. In the shearing interferometry which is made possible in this way, primarily different locations of the pupil of the imaging system are compared with one another interferometrically, for example by the light of a zeroth order of diffraction which has traversed the diffraction grating undiffracted having superimposed on it the light of the first orders of diffraction.
The wavefront source used can be a wavefront module which comprises a perforated mask, arranged in the object plane, with a two-dimensional hole structure, refractive, diffractive or reflective focusing optics connected upstream of the perforated mask in order to concentrate incident illuminating light onto the holes of the perforated mask, and a matt disk connected upstream of the focusing optics in order to homogenize the intensity distribution in the mask plane and, if appropriate, to reduce the spatial coherence. As a result of using such wavefront modules, it is possible to use the illumination system of a projection exposure installation at the same time as an illumination system for the interferometric measuring device, in order for example to check the imaging quality of the projection objective from time to time at the location at which the projection exposure installation is used. For this purpose, in each case replacement of the reticle provided for the chip production operation and having the useful patterns representing the chip structure by such a wavefront module is necessary.